1. Field of the Invention
The present invention relates to imparting spin to optical fiber, and more particularly to imparting spin to an optical fiber while the fiber is being drawn.
2. Technical Background
Light traveling in an optical fiber has two polarizations. For optical fibers that are perfectly circularly symmetric in both geometry and internal and applied stress, operation at a wavelength or in a wavelength range which is regarded as “single-moded” actually supports two orthogonal polarization modes, wherein the two polarization modes are degenerate, propagating with the same group velocity and having no time delay difference after traveling the same distance in the fiber. However, in practice, an optical fiber is not perfectly circularly symmetric. For example, imperfections such as geometric and form deformation and stress asymmetry break the degeneracy of the two modes. See, for example, Rashleigh, S.C., Journal of Lightwave Technology, LT-1:312-331, 1983. As a result, the two polarization modes propagate with different propagation constants β1 and β2. The difference between the propagation constants is termed birefringence Δβ, the magnitude of the birefringence being given by the difference in the propagation constants of the two orthogonal modes:Δβ=β1−β2  (1)
Birefringence causes the polarization state of light propagating in the fiber to evolve periodically along the length of the fiber. The distance required for the polarization to return to its original state is the fiber beat length LB, which is inversely proportional to the fiber birefringence. In particular, the beat length LB is given by:
                              L          B                =                              2            ⁢            π                                Δ            ⁢                                                  ⁢            β                                              (        2        )            
Accordingly, fibers with more birefringence have shorter beat lengths and vice versa. Commercial fibers exhibit a wide variety of beat lengths since the geometric and stress asymmetries of such fibers vary along the length of the fiber and between different fibers. Typical beat lengths observed in practice range from as short as 2-3 millimeters (a high birefringence fiber) to as long as 10-50 meters (a low birefringence fiber).
In addition to causing periodic changes in the polarization state of light traveling in a fiber, the presence of birefringence means that the two polarization modes travel at different group velocities, the difference increasing as the birefringence increases. The differential time delay or differential group delay (DGD) between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion, and thus PMD is very detrimental in high bit rate systems and analog communication systems. For a uniform linear birefringent fiber without perturbation, i.e. externally imposed perturbation, the PMD of the fiber typically grows linearly as the fiber length increases. However, in a longer length, random mode coupling is inevitably introduced into the fiber due to externally imposed perturbations, the PMD increase along the fiber is thus proportional to the square-root of the fiber length statistically.
Various attempts to reduce PMD have been made. Known methods of reducing PMD involves spinning the preform and/or the fiber during the fiber drawing process. The spinning causes the birefringence axis of the fiber to rotate along the fiber. By performing the spinning during drawing, i.e., when fiber is being drawn from a heated portion of the preform, the spin imparted to the fiber will tend to stay in the fiber even when the fiber is cooled. As used herein, “spinning” or “applying a spin” are distinguished from “twisting” the fiber during the post-draw stage after the fiber has cooled to temperatures substantially lower than the temperature of the fiber during draw. For example, twisted fibers will generally relax to a non-twisted state once an applied torque is released. In addition, due to mechanical stress, twisted fibers possess an elastic-optic effect making their PMD properties dramatically different from spun fibers.
Sinusoidal spin profiles have been used extensively to reduce the PMD of optical fibers. Currently, in production processes, the fiber PMD can be improved by a factor of 2 to 5 using such spin profiles, although much better PMD reduction is predicted by using certain spin parameters. To achieve a high degree of PMD reduction, the spin magnitude and spin period need to be precisely controlled. In known spin devices, the spin magnitude typically has a fluctuation of about ±0.5 turns/m from the specified spin profiles, wherein the resultant PMD is actually an average PMD over the region of fluctuation. On the other hand, sinusoidal spin profiles are most effective for optical fibers with relatively small intrinsic birefringence, typically fibers having a beatlength larger than a few meters. For optical fibers having a relatively large intrinsic birefringence, the PMD reduction from a specific spin profile is beatlength dependent. Furthermore, increasingly better PMD reduction is difficult to achieve because, for example, the fiber intrinsic birefringence is typically not uniform and/or the spin magnitude typically can not be controlled precisely enough.
A spatial spin function can be detected in a finished optical fiber by, for example, using near-field resonant backscattered light. See A. Ashkin et al., Applied Optics, Vol. 20, pages 2299-2303, 1981.